1. Field of the Invention
The present invention relates to an optical beam scanning method and apparatus for scanning the scanning surface with an optical beam and an image forming method and apparatus for forming images by exposing a photosensitive material with scanning with an optical beam, and particularly to an optical beam scanning method and apparatus which have enhanced resolution in the sub-scanning direction and an image forming method and apparatus for enhancing resolution in the sub-scanning direction without generation of ununiform distribution of exposure to realize high precision and high quality image forming.
2. Description of the Related Art
As an image forming apparatus which forms images using an optical beam such as a digital copying machine and laser printer, apparatuses are known in which an optical beam modulated in accordance with image information is reflected for deflection by an optical deflector, for example, a polygon mirror and the scanning surface such as a photosensitive material surface is scanned with such an optical beam to record image information.
However, since the optical beam scanning line on the scanning surface is discrete in the period of the main scanning in such an image forming apparatus, there rises problems such as lower drawing accuracy in the sub-scanning direction, for example, lower resolution in the sub-scanning direction, generation of jagged oblique lines (jaggy), lower reproducibility of fine lines. Therefore, it is difficult to provide high precision character and line images required in the field of the printing and desktop publishing.
Meanwhile, resolution in the sub-scanning direction can be improved through high speed main scanning by rotating a polygon mirror at a high speed, but it has been difficult to simultaneously realize high speed main scanning and high resolution because there is a limitation in mechanical drive to attain the high speed rotation of the polygon mirror.
Therefore, in view of overcoming such problems, the system for realizing zigzag scanning by an optical beam has been proposed, for example, in Japanese Unexamined Patent Publication Nos. Sho 64-32691 (1989) and Hei 3-131818 (1991).
In this optical beam scanning system, when the optical beam is used for scanning in the main scanning direction, the optical beam is repeatedly displaced only in a very short distance (for example, distance equal to the sub-scanning pitch or one-half of the distance) in the sub-scanning direction using an optical beam defection element such as a piezoelectric element and electro-optical element, the loci of the zigzag optical beam scannings are respectively formed on the main scanning lines (N, N+1, . . . ) on the scanning surface as shown in FIG. 26.
In this optical beam scanning system, when the optical beam is used for scanning in the sub-scanning direction, the regions between the adjacent main scanning lines can also be scanned and therefore resolution in the sub-scanning direction can be enhanced and the drawing accuracy, as a result, in the sub-scanning direction can be improved.
However, the optical beam scanning system of the related art has a problem that since a difference in the scanning rates at the internal and external sides of the optical beam is generated at each displacement point in the sub-scanning direction, namely at each bending part, ununiform distribution of exposure is generated at such displacement points. FIG. 27 shows an exposed image appearing when the optical beam is used for zigzag scanning in a pitch P/2 which is a one-half of the sub-scanning pitch P. In this figure, the amount of exposure increases at internal regions 32A of pixels 32 corresponding to the bending parts because the scanning rate is rather lower, while the amount of exposure reduces at external regions 32B because the scanning rate is rather fast. Thereby, ununiformity is generated in distribution of exposure and the areal modulation of exposed images faithful to concentration information and edge information of pixels is disabled, resulting in the disadvantage that high precision and high quality image forming is impossible. This disadvantage also occurs, for example, in an optical beam scanning apparatus for image reading.